Fitness is a complex parameter which is determined by many factors in the life-history of a genotype. One way to measure the fitness of a genotype will be to make use of Fisher's Malthusian parameter m, which is defined as a real root of the equation, where 1(x) is the survival rate of a genotype to age x, f(x) is the fecundity at age x, t is the age when the genotype starts to produce progeny and T is the age when the genotype terminates reproduction. In Drosophila, T contributes only little in determining m so that 1(x), f(x) and t may be considered as 3 major factors in the life-history. Using Drosophila melanogaster as an experimental material, the present study attempts to examine the relationships among these components of fitness for various inbred lines of various origins as well as for wild strains from natural populations. It is also attempted to example the effects of environmental conditions (temperature) upon each component of fitness. Investigations will be conducted at different temperature conditions for not only those strains mentioned above but also the artificially synthesized milk-bottle populations.